Grief can do strange things to people and everybody processes it in their own way.
She thinks its unacceptable and that doing so makes you a bad person.
We are complicated creatures with complex emotions and things are rarely as simple as good or bad.

Ikon Republik
They were bored
Okay, this is arguably the dumbest reason for cheating.
Dont just go bang the next person you see simply because you couldnt think of anything better to do.
However, to some, this seems to be a legit reason to be unfaithful.
Revenge
Your partner has cheated on you and youre just evening the score.
Sleeping with someone else might give you the validation youre craving.
Loneliness
Your partner might not be giving you what you need.
Or maybe the relationship is long distance and you miss being intimate with someone.
Its not about your partner, its about you.
Its about filling the hole they are leaving in your life.
Maybe your partner has been spending too much time working and too little time on you.
Maybe you are struggling with something and they just cant see it.
Cheating on them can be a cry for help, a cry for attention.
They just dont care
Some people just dont take fidelity very seriously (or at all).
Grief
Grief can do strange things to people and everybody processes it in their own way.
You dont want to talk or discuss or even be lovingly held.
You just want to feel and remember that you are alive.
Sex, particularly with someone who isnt your partner, could be the answer to that.
Alternatively, some people will look for what they are lacking in their relationship elsewhere.
Self-sabotage
Sometimes, we do things to hurt ourselves.